A guy I used to work for used a smallish oxy-acet welding tip to
weld thick (.200") sections of sterling presentation sculptures
together.

Tricky, but it worked. The trick was to get the flame hot as hell,
get a lot of thick paste flux into the area, and then get in, melt
your target, and get back out again fast, before the heat had time to
migrate anywhere. As I recall it, he didn’t do running seams, just
spots. If he needed a running weld, he’d do it in chunks.

As I said, it was tricky, but I can’t imagine a silver thermite
would be any less tricky. (Orders of magnitude more tricky, at a
guess.)

The tip was something like a 0 or a 00. A for-real steel tip, not a
jeweler’s tip.